Cells respond to diverse extracellular stimuli by activating specific signal transduction pathways. It is both the exquisite specificity of these responses, and in some instances the redundancy of the responses that are critical in the regulation of proliferation, apoptosis, immune response, and survival. The objective of the proposed research is to understand the molecular mechanisms that regulate a signal transduction pathway from a cytokine cell surface receptor to the nucleus. The cytokines bind to receptors that are associated with cytoplasmic tyrosine kinases of the Janus kinase (JAK) family. Following ligand binding the JAKs are activated and phosphorylate the receptors and transcription factors in the cytoplasm called signal transducers and activators of transcription (STATs). The tyrosine phosphorylated STATs translocate to the nucleus to bind specific DNA recognition sites in the promoters of responsive genes. The actions of STATs direct new gene expression and specific biological effects in response to the cytokine. The goals of this proposal are to reveal common mechanisms that govern the STAT family of transcription factors, and in a complementary approach, to reveal properties that distinguish individual STATs and direct specific physiological effects. Cellular localization will be investigated as a shared regulatory mechanism that is critical to the response and action of the STATs. Molecular biology approaches and genetic approaches will be used to identify regulatory domains within the STATs that direct cellular localization. The specificity of individual STATs in both response and action will be investigated by evaluating the effects of hybrid STAT proteins, and by evaluating the cross-talk of particular STAT proteins with distinct signal pathways of the cell.